Sky Lights

Five astronomy illusions to trick an unwary autumn sky observer

Dust clouds blot out light from the center of our galaxy, seen over Las Campanas Observatory in Chile.Photograph by Roger Ressmeyer/Corbis.

This month of Halloween brings more than children's pranks. In October the sky offers a few tricks of its own.

TRICK 1The frozen constellations. Over the course of a year, constellations appear to shift steadily to the west. This movement, caused by Earth's orbit around the sun, means that stars rise four minutes earlier each day, or two hours earlier each month: A constellation that rises at 9 p.m. today will rise at 7 p.m. a month from now. Nightfall therefore brings a slightly different set of stars into view from one day to the nextbut not now. During late summer and the first several weeks of autumn, the heavens seem nearly frozen in place. The reason is simple yet subtle. New stars still rise four minutes earlier, but sunset occurs earlier too. If you, like most casual stargazers, go out to look when the sky turns dark, the constellations seem to slide westward at half their average rate.

TRICK 2The hole in the Milky Way. Take a look at the ghostly band of the Milky Way, optimally situated in the sky this month. Nearly overhead you will see the Great Rift, a section where the light of our galaxy is strangely missing. Noted since antiquity, this gap is where the mythological "river of milk" divides into two parallel streams. No rift actually exists. The stars of the Milky Way are packed as tightly there as anywhere else, but vast dark clouds of dusty hydrogen block their light. This giant nebula contains roughly 1 million times the mass of the sun. It looks like a hole only because human vision tends to see dark patches as gaps.

TRICK 3The flattened sky. The universe stretches for billions of light-years in every direction, so the sky should look like a perfect hemisphere, as distant overhead as it is at the horizon. Does it? You don't need to wait until dark to check it out; any cloudless day will do. Notice that the overhead sky seems squashed, much closer to you than the areas near the horizon. The effect is caused in part by our lifetime visual experience with clouds. Overhead clouds really are closer. There is also a natural inclination to relate the horizon to the foreground objects on Earth, which you can see are quite distant. When you look straight up, there is no point of reference for you to gauge distance, so that part of the sky looks nearer.

TRICK 4The giant rising moon. The phantom flattening of the sky is especially powerful when the moon lies right on the horizon. The October 10 hunter's moon, for instance, will seem much bigger during its dusk rising than it will later in the night. The moon should actually appear a bit smaller when near the horizon, because it is slightly farther away. This well-known moon illusion applies to star patterns as well. This month the Big Dipper lurks at its lowest point of the year, scraping the northern horizon. The result is startling: The Dipper's familiar shape appears so enormous that it can be hard to recognize.

TRICK 5The vanishing colors of fall. The vivid yellows, oranges, and reds of fall foliage are vivid only during the day. At night, away from artificial lights, they are replaced with drab green or gray. This happens because the retina loses its ability to detect color in extremely low light. In semi-darkness, green is the main color we can still see, which is why many cities have changed the colors of their fire engines from red to chartreuse. Likewise, bright stars such as Orion's Betelgeuse and Rigel, rising at midnight, are intense enough to display strong colors such as oranges and blues but dimmer stars, such as those of Orion's head, appear white to the unaided eye.

There is a simple way to turn this heavenly trick back into a treat. Peer through a telescope or a pair of binoculars and suddenly hundreds of stars will look bright enough to show their true colors.

The Grand Illusions Web site (www.grand-illusions.com) describes many clever visual tricks and optical illusions, including the "moon illusion" that makes the moon appear huge when it's close to the horizon.